We propose to continue our research on the crystal structures of small biologically important molecules, with an emphasis on studies that will enhance our knowledge of the nature of intermolecular forces and their relationship to molecular geometry and reactivity. Of particular interest to us are interactions involving hydrogen atoms, which cover most of the surface area of biological molecules and hence must be of primary importance in the reactions of biological systems. In efforts to increase the accuracy with which the electron distribution associated with hydrogen atoms can be determined, we shall analyze X-ray diffraction data collected from crystals of simple molecules, such as glycine and L-alanine, at both room temperature and at temperatures down to 20K. We shall make special efforts to obtain data of the highest possible quality, and to eliminate or correct for any experimental errors that may arise. In particular, in the coming year we hope to come to a clear understanding of the problems associated with truncation errors that arise at the high scattering angles we are using. The electron distribution effects we are looking for are small, and cannot be examined with confidence unless the data are of exceptionally high quality.